Chassis design began as a direct follow-through from horse-pulled carriages. In the early years of motoring it became common for the manufacturer to provide a rolling chassis, and a coachbuilder to provide the body. From both of these came the ladder frame. Variations on a theme (different section types for the longmembers, members bent out fo true in the lateral and vertical planes), but ultimately there was still a basic chassis upon which separte bodies could be fitted. For this reason the ladder frame remains popular with trucks and cabin-only vans.

It was post-WWII that a few key engineering rules were truly adopted.- The chassis should be as stiff as practicable - let the suspension do all the suspension work.- Section is everything for chassis stiffness - two members spaced apart will be stiffer than one large member, a large section member will be stiffer than a shallow section member.- Light weight means better performance

Early post-war racers were still generally ladder-framed. The first significant variation was the 4-tube design - two upper tubes, two lower tubes, connected at key points by bulkhead assemblies. Connecting straps in between were quickly added (and soon replaced by tubes), but there was no triangulation so it was not a spaceframe. The breakthrough chassis (not necessarily the first) was the Cooper Mk VI of 1952.

I'm not sure who first introduced diagonal tubes (originally just in the vertical planes) to the 4-tube design, but this was the first effective spaceframe. Of course it was not a true spaceframe as it still relied on primary tubes (unlike the birdcage Maseratis).

In time, the triangulation tubes would be replaced with panels (bonded or rivetted), offering more stiffness for less weight. In a similar manner, some used the body panels as a semi-stressed skin for added strength.

An interesting variation on the 4-tube design is the bent-tube design (introduced on the Cooper Mk VIII in 1954 and used for many years by Cooper). Here, the four tubes curved along their entire length. The benefit was a much smaller frontal area that could carry a far more aerodynamic body. The Monaco, Bobtail and Jack Brabham's 1959 World Champion car all used the same basic chassis.

Finally you have the unibody design, removing the tubular frame and leaving just the panels. Strictly speaking, this is not a monocoque which is literally a single shell - a unibody can use a series of box structures. A modern F1 chassis is much closer to a true moncoque than say a Lotus 49, and it's very hard to build a monocoque with doors. The breakthrough 'monocoque' chassis was the Lotus 25 of 1962, although the Killeen and Trimax had adopted the concept all the way back in 1949 - the creator of the Killeen had even written to Colin Chapman offering his ideas, only to be told that Colin saw no benefit!

A backbone chassis, which can be tubular or a complex pressed unit is something of a compromise. As used on the original Lotus Elite, and of coure TVR, it provides a decent structure for a 2-seat car where it is difficult to include structure at the sides (in both the above cases because of a grp body that you would rather not stress). The downside is that you are giving up a huge amount of section across the car, costing torsional stiffness expecially, and which you need to recover by adding structure in the central core. It has its merits, but is less than optimal for competition.

And those are the basic chassis layouts. The fun starts with the hybrid systems, such as:

One point to bear in mind is that a lot of misinformation is spread about the various merits of the chassis types. This is made worse by the fact that in the real world you rarely get easily pigeonholed chassis. For example most spaceframes will have some sections that aren't fully triangulated or that rely on panelling to give them strength. So you end up with ladder/spaceframe/monocoque hybrids.

The ladder frame came first

Then this was made partly 3D into a multitube frame

Then came the spaceframe

Then came the backbone chassis. The backbone can be a spaceframe, multitube, ladder or monocoque but it is always a central structure down the middle of the car (TVR and Lotus Esprit being examples)

In mass manufacture the monocoque quickly became the default design. Steel is most common, composites are used and plywood has actually made some very good chassis (early Marcos).

However....

The aluminium spaceframes produced by some mass makes are actually not triangulated so technically are at best multitubes or ladder frames

The huge advantage of a spaceframe over a ladder frame is really only relevant to racing. On a road car it probably makes around 5 percent difference for an equally well designed chassis. The big differences that many will claim are based on two things (1) comparing hopeless open channel ladder with spaceframes designed by Chapman or (2) people who don't actually have any analysis or testing to back up their claims.

Which is best...

Tricky. The small sportscar business has relied heavily on the backbone for decades. In cheap racing (i.e. kitcars) it's mainly spaceframes. Mass production has long depended on monocoques. Ladders are probably a bit ignored. For a budget road car they can be just fine.

One minor correction - the original Elite didn't use a backbone chassis; it was as close as you'll get with a car with doors to a true monocoque (in glassfibre, in the Elite's case). The backbone chassis was introduced with the Elan, though the later 'wedge' Elites used backbones, too.

Cymtriks and I have debated the ladder frame vs. spaceframe argument many times. Whilst Cymtriks is correct in stating that a ladder frame is adequate for a budget road car, it's worth pointing out that his suggestion that anyone arguing with him is comparing a sooper-dooper top-notch spaceframe with an unboxed Austin Seven chassis is rather disingenuous.

The 'Seven' spaceframe, either in original Chapman form or later Caterham developed form, is a very compromised example of the type (mainly because of the lack of 3-dimensional triangulation in the cockpit bay area which, ironically, means that it performs much like a deep-section ladder frame would). To be fair to Chapman, he was an extremely pragmatic engineer who knew how good was 'good enough', and he designed the Seven to be stiff enough to work with the relatively supple suspension and light weight of the orignal Seven, whilst being cheap and easy to manufacture. The torsional stiffness of the original Chapman Seven was considerably less than 1,000 lb.ft/degree, which would be considered absolutely dire by modern standards.

Cymtrik's figure of only 5% worse comes from comparing a badly designed copy of a 'Seven' chassis (stiffer - but also much heavier - than the Chapman original) against a well designed ladder frame.

If you want to see a better (in terms of stiffness:weight) spaceframe, you need to look at some of the purer Chapman designs (eg. the Lotus Mk. 8 - still relatively lousy stiffness but very light - or single seaters), Frank Costin's designs, or some of the modern spaceframe race cars like the later Mallocks or Formula Fords.

But a good spaceframe is a very labour-intensive (hence costly) thing to produce, so there's sense in Cymtrik's suggestion that a ladder frame will do the job for a budget car.

As HiRich says, the complications come with the 'hybrid' chassis forms... the argument over what sort of chassis lies under the Lotus Elise could go on forever!

One minor correction - the original Elite didn't use a backbone chassis; it was as close as you'll get with a car with doors to a true monocoque (in glassfibre, in the Elite's case). The backbone chassis was introduced with the Elan, though the later 'wedge' Elites used backbones, too.

D'oh! You are quite correct.

Having worked with ladder frames - historic and 'modern', I wouldn't consider it viable any more, unless the vehicle concept led to it (e.g. the cab-only van mentioned earler, where you don't know what body will go on it). Torsional stiffness will be very low with so little section to work with, and you end up with a lot of weight just to maintain bending strength. At least, I would look in the van's case to plating over the entire structure (a single panel over long- & crossmembers, not just top-hat sections), and probably underneath as well.

Having worked with ladder frames - historic and 'modern', I wouldn't consider it viable any more, unless the vehicle concept led to it (e.g. the cab-only van mentioned earler, where you don't know what body will go on it). Torsional stiffness will be very low with so little section to work with, and you end up with a lot of weight just to maintain bending strength. At least, I would look in the van's case to plating over the entire structure (a single panel over long- & crossmembers, not just top-hat sections), and probably underneath as well.

Goodness gracious me you'd better tell that to Dax, BRA, NG and Moss all of whom have made money out of selling ladder frame suported chassis.

Torsional stiffness is only much lower when the channels are "C" or "Z" section or the frame is not "X" or "K" braced. A rectangular section tube frame with proper bracing is much better. A simple ladder frame in 4x2-14g with front and rear laterals and a central X brace is actually lighter and stiffer than the popular Locost (Ron Champion's book) design.

Taking an equally well designed frame I reckon that the weight penalty is circa 30 to 40Kg, i.e. a 150lb spaceframe is roughly equalled by a 220lb ladderframe. On a race car that's a lot but on a road car where longevity, economy of build and access matter a bit more and speed a bit less the gap may well be acceptable. The weight penalty could be reduced by carefull design of the bodywork, possibly using the time and money saved in the chassis to move to a lighter composite or better thought out thickness for it.

As I said, I have seen first hand the issues with ladder frames, on both the drawing board and MIRA R&H track (also the Belgian pave). And as I said at the top, section is everything - if you have the opportunity to create a structure over feet, why restrict your self to a few inches?

None of the companies you mention (including Locost) are exactly famed for their chassis design. Those that are abandoned the ladder a long time ago.

Goodness gracious me you'd better tell that to Dax, BRA, NG and Moss all of whom have made money out of selling ladder frame suported chassis.

3 out of 4 of whom are no longer trading, strangely.

Cobras; weight hardly matters, since the engines on any semi-authentic Cobra will have enough grunt to drag anything along at a reasonable rate of knots and lairy handling merely adds to the authenticity.

cymtriks said:

Taking an equally well designed frame I reckon that the weight penalty is circa 30 to 40Kg, i.e. a 150lb spaceframe is roughly equalled by a 220lb ladderframe.

..yet you are promoting ladder frames as a sensible solution for 'budget' cars?

Remind me to put a hundredweight of spuds in my boot next time I'm down the farmer's market, as a means of improving my fuel consumption (not to mention performance with a small, fuel-efficient engine).

As I said, I have seen first hand the issues with ladder frames, on both the drawing board and MIRA R&H track (also the Belgian pave). And as I said at the top, section is everything - if you have the opportunity to create a structure over feet, why restrict your self to a few inches?

None of the companies you mention (including Locost) are exactly famed for their chassis design. Those that are abandoned the ladder a long time ago.

But are these issues really down to the chassis type?

The only common ladder frames around are open channel designs on vans and trucks or box section designs under off road vehicles. These are hardly relevant or comparable in any way to something like the cars I mentioned.Both will have very different suspensions and much higher centres of gravity for a start.

Regarding issues on the drawing board, what issues? On cost the ladder wins. For longevity and access (both for occupants and for the mechanicals) it probably wins.

I agree that it has virtually become extinct but I would suggest that this is because the backbone chassis gives all the above advantages in addition to light weight. Saying that no one making the ladder frame proves it must be bad ignores the equally true observation that no production car (except for LSIS types) use the spaceframe either.

Arguing that a chassis type has problems also fails to consider the advantages. Think of it like this:A monocoque with a hatchback is bad for stiffnessA monocoque with four doors is bad for stiffnessSteel rustsSteel monocoques are not the lightest construction methodFront wheel drive is bad for handlingFront transverse engines are bad for weight distributionSteel press tooling is very expensiveSo why do cars that have these features dominate the roads? Because that list of faults ignores the list of advantages.

Goodness gracious me you'd better tell that to Dax, BRA, NG and Moss all of whom have made money out of selling ladder frame suported chassis.

3 out of 4 of whom are no longer trading, strangely.

All still trading except Moss. NG have been renamed as Findhorn though and BRA no longer make the 289 replica.

Sam_68 said:

Cobras; weight hardly matters, since the engines on any semi-authentic Cobra will have enough grunt to drag anything along at a reasonable rate of knots and lairy handling merely adds to the authenticity.

I would expect that the lairy handling is more to do with a lot of power in a very short wheelbase. Are you aware of a spaceframed cobra (or similar power/wheelbase combination that handles without being lairy?

Sam_68 said:

cymtriks said:

Taking an equally well designed frame I reckon that the weight penalty is circa 30 to 40Kg, i.e. a 150lb spaceframe is roughly equalled by a 220lb ladderframe.

..yet you are promoting ladder frames as a sensible solution for 'budget' cars?

Remind me to put a hundredweight of spuds in my boot next time I'm down the farmer's market, as a means of improving my fuel consumption (not to mention performance with a small, fuel-efficient engine).

I'm not really promoting it, more giving it a fair trial. Most of its opponents ignore the advantages of cost, simplicity and access and just talk about the weight of race track cars or the miserable stiffness of open chanel sections which no one today would use anyway.

As I've said before the difference for the finished car is around 5 to 10 percent. Lets say its 8 and that the weight is 4 percent more and the stiffness 4 percent less. You probably won't notice 4 percent less stiffness and an extra 100cc will make up the small weight penalty on most most cars. Now add the advantages of easier access cost and longevity. You are correct about fuel economy, but will you notice the combined effect of an extra 30 Kg and 100cc? Thinking like this won't win a race but it might very well produce a perfectly acceptable road car a bit like the NG TC or any number of Cobra replicas.

The only common ladder frames around are open channel designs on vans and trucks or box section designs under off road vehicles.

Do you not think there's a reason for that?

cymtriks said:

These are hardly relevant or comparable in any way to something like the cars I mentioned.

Quite right.

Trucks and 4x4's don't need to handle well, structure weight is virtually irrelevant because it is such a small proportion of overall weight/payload, and they're not even slightly worried about stiffness related issues of refinement such as scuttle shake.

The reason that there's nothing comparable to something like the cars you mention is that every professional automotive engineer out there has long since recognised that for the types of cars you mention, the disadvantages of ladder frames outweight the advantages, even where economy is paramount.

But you never know... perhaps Orwell was right about being a minority of one?

Are you aware of a spaceframed cobra (or similar power/wheelbase combination) that handles without being lairy?

Yes. The GD427 (TVR-style backbone spaceframe) is generally recognised as the best handling Cobra replica out there and it's certainly the only one I've driven that doesn't scare me stless.

Then there was the Adrian Reynard influenced RAM Cobra; again noted for its better-than-average handling, though I've not driven one personally.

Of course you don't get many true spaceframe Cobras because the basic design of the car isn't suited to them (door openings etc.), but there are plenty of other spaceframe and monocoque cars of similar (and greater) power and similar proportions that handle just fine.

cymtriks said:

I'm not really promoting it, more giving it a fair trial.

Really? For years, at every opportunity, you've trotted out the same stale arguments in favour of ladder frames and have only acknowledged that they may have shortcomings the duress of prolonged debate. Sounds like promoting it to me...

Like I said: every automotive design engineer has weighed the advantages and disadvantages of the ladder frame and has rejected it.

Any sensible engineer will try to select a design solution because it offers the best compromnise, not because they think that it's not really so much worse than the alternatives when you think about it, if you give it the benefit of the doubt and turn a blind eye to the more serious deficiencies.

Longtitudinal bending strength: Working within a section of 4-6" is never going to compare with 12" section (e.g. caterham) or 48" section (most production cars). You can expect to use a thicker gauge, adding weight & unit cost. You are likely to need strengthening liners at critical points.

Add these two together, and you have added a lot of 'junk' that adds weight , unit and assembly costs.

Centre of Gravity: Member depth will be defind by bending strength, forcing the floor (and many critical systems) above this level. Other chassis designs place a much smaller section below the floor, as that above it takes much of the strain. So the ladder chassis is likely to raise the CoG

Ladder manufacture: The members could be formed by folding, (lower tooling cost), but only if they are straight and uniform. This will inevitably lead to significant lengths being over-engineered. Any change in section or profile means they have to be pressed anyway, negating that benefit, but still carrying the over-engineering. Even straight members would reguire at least one press run to punch the various holes.

Underfloor access: If you go with single section members you will have a large structure interrupting the paths of some major systems: exhaust, driveshafts, steering. This leads you towards raised floor (& CoG), and reduced clearance. Much (but not all) of this can be solved with heavily profiled members (likely to require multiple press passes). The way other chassis systems allow you to 'move' the metal in and out of places much more easily makes the design far less of a compromise.

Tooling & assembly costs: With the ladder design, you are still going to need a large number of non-bodyskin panels - bulkheads, inner pillars, etc.whether in pressed steel or grp. Cost savings are limited. 'Better' design to improve the load-bearing properties of these elements adds to the tooling costs, but will add only a handful of press passes on the complete vehicle - quite possibly fewer than the number of work incurred in creating those long- and crossmembers for the ladder itself. In volume production, the expense of tooling large structural panels (e.g. body side: A,B,C pillars sill & roof edge) will quickly be compensated by the reduced assembly costs - reduced jigging and far more accurate dimensional consistency can be considered a free bonus.

Impact strength: Long members are certainly capable of making good crumple zones for direct front & rear impacts (though inevitably as bolt/weld-on structures, negating the cost benefits). Less so for standard side impacts. Low-volume manufacturers may ignore this, but not volume. Other chassis systems are placing structural material in the very place where impact strength is required.

All of which says "why bother with a ladder frame?". Other designs allow you to place material where you need it (and remove it where you don't. Strength and stiffness are much easier to achieve within cost & weight targets. It may require more design time, but that's a small part of the vehicle cost. On like-for-like concepts (be it Caterham or Mondeo), you can expect the ladder design to lag on all performance criteria (strength, stiffness, weight, impact on the performance of other systems). It might prove cheaper on tooling or panel forming or assembly, but it's unlikely to deliver on all three.An integrated design, where the entire structure performs the chassis role, makes far more sense, unless the core concept requires mountable bodies (so vans, trucks & PSVs as already explained). The appropriate design tools (3D CAD, FEA) are now readily available to low and volume manufacturers alike. And the vast majority of manufacturers across the spectrum seem to agree.

Longtitudinal bending strength: Working within a section of 4-6" is never going to compare with 12" section (e.g. caterham) or 48" section (most production cars). You can expect to use a thicker gauge, adding weight & unit cost. You are likely to need strengthening liners at critical points.

Add these two together, and you have added a lot of 'junk' that adds weight , unit and assembly costs.

Centre of Gravity: Member depth will be defind by bending strength, forcing the floor (and many critical systems) above this level. Other chassis designs place a much smaller section below the floor, as that above it takes much of the strain. So the ladder chassis is likely to raise the CoG

Ladder manufacture: The members could be formed by folding, (lower tooling cost), but only if they are straight and uniform. This will inevitably lead to significant lengths being over-engineered. Any change in section or profile means they have to be pressed anyway, negating that benefit, but still carrying the over-engineering. Even straight members would reguire at least one press run to punch the various holes.

Underfloor access: If you go with single section members you will have a large structure interrupting the paths of some major systems: exhaust, driveshafts, steering. This leads you towards raised floor (& CoG), and reduced clearance. Much (but not all) of this can be solved with heavily profiled members (likely to require multiple press passes). The way other chassis systems allow you to 'move' the metal in and out of places much more easily makes the design far less of a compromise.

Tooling & assembly costs: With the ladder design, you are still going to need a large number of non-bodyskin panels - bulkheads, inner pillars, etc.whether in pressed steel or grp. Cost savings are limited. 'Better' design to improve the load-bearing properties of these elements adds to the tooling costs, but will add only a handful of press passes on the complete vehicle - quite possibly fewer than the number of work incurred in creating those long- and crossmembers for the ladder itself. In volume production, the expense of tooling large structural panels (e.g. body side: A,B,C pillars sill & roof edge) will quickly be compensated by the reduced assembly costs - reduced jigging and far more accurate dimensional consistency can be considered a free bonus.

Impact strength: Long members are certainly capable of making good crumple zones for direct front & rear impacts (though inevitably as bolt/weld-on structures, negating the cost benefits). Less so for standard side impacts. Low-volume manufacturers may ignore this, but not volume. Other chassis systems are placing structural material in the very place where impact strength is required.

All of which says "why bother with a ladder frame?". Other designs allow you to place material where you need it (and remove it where you don't. Strength and stiffness are much easier to achieve within cost & weight targets. It may require more design time, but that's a small part of the vehicle cost. On like-for-like concepts (be it Caterham or Mondeo), you can expect the ladder design to lag on all performance criteria (strength, stiffness, weight, impact on the performance of other systems). It might prove cheaper on tooling or panel forming or assembly, but it's unlikely to deliver on all three.An integrated design, where the entire structure performs the chassis role, makes far more sense, unless the core concept requires mountable bodies (so vans, trucks & PSVs as already explained). The appropriate design tools (3D CAD, FEA) are now readily available to low and volume manufacturers alike. And the vast majority of manufacturers across the spectrum seem to agree.

Your arguments seem to jump from kit cars to mass produced cars to trucks.

Some of your points are perfectly correct for a mass produced car. However for something like a kit car most of your comments about body panels and press tooling are irrelevant. Your comment about a higher CoG is wrong if a separate chassis is assumed.

If I was given a design brief for a kit car to win a race I'd design a composite monocoque with no doors and possibly a steel subframe at each end to support the driveline and suspension.

If I was given a design brief to create a kit car for fast road use or weekend racing I'd use a backbone or spaceframe.

If I was given a design brief to create a simple road going kit car I'd consider an X braced ladder frame.

Volume and marketing are the true influences however. All of the above change completely if zeros are added to the production volumes.

Are you aware of a spaceframed cobra (or similar power/wheelbase combination) that handles without being lairy?

Yes. The GD427 (TVR-style backbone spaceframe) is generally recognised as the best handling Cobra replica out there and it's certainly the only one I've driven that doesn't scare me stless.

Then there was the Adrian Reynard influenced RAM Cobra; again noted for its better-than-average handling, though I've not driven one personally.

Do you think this difference is actually due to the chassis or more due to the suspension set up?

The GD has bespoke parts and geometry in its suspension as opposed to Jaguar parts. Perhaps the suspension is the key? The RAM wasn't realy a replica, more a look-alike. Its body was lower than a visual replica and lighter compared to other Cobras aswell IIRC. Perhaps this shift in focus from making a replica to making a performance car with a similar look was the key, not the chassis type.

I can't think of many cars that have that much power and torque in a wheelbase of only 90 inches. There's the TVR Griffith and the pre 996 911 turbo but these also had a reputation for being tricky.

If I was given a design brief to create a simple road going kit car I'd consider an X braced ladder frame.

Narrowing down your design criteria to a low budget, low volume car is counter-intuitive: to stand the remotest chance of commercial success, low budget has, almost by definition, got to mean high volume.

You can't even argue that volume is irrelevant for one-off budget self-builds, since in that case the labour-intensiveness of spaceframe fabrication ceases to be a major factor and, again, the ladder frame falls out of the picture.

As such, you're basically suggesting that the ladder frame remains the optimum design solution for a problem that doesn't exist.

cymtriks said:

Do you think this difference is actually due to the chassis or more due to the suspension set up?

As you are perfectly well aware, the design of any car is a very complex compromise, so it's difficult to isolate a single factor and say that it's entirely responsible for a fundamental shortcoming of an entire group of cars with widely differing specifications.

If you're intent on playing those sorts of games with semantics, I could equally ask you, since the GD427 is generally recognised as the best handling Cobra replica by far: do you think that the inferiority of all the many ladder frame replicas is due to the suspension set-up and, if so, why do you think this should be the case?

FWIIW, as an ex-owner of a Griffith 500, I'm convinced that the really tricky and unpredictable bit of its handling is due to flaws in the rear suspension geometry and damping. Most of the time it was perfectly benign and manageable, provided you didn't do anything stupid: it only seemed to be specific situations (which were unfortunately difficult to predict in themselves) that resulted in the chassis becoming severely and dangerously unsettled.